U.S. patent number 5,157,525 [Application Number 07/601,136] was granted by the patent office on 1992-10-20 for control of liquid crystal display visual properties to compensate for variation in the characteristics of the liquid crystal.
This patent grant is currently assigned to EEV Limited. Invention is credited to Timothy J. Eaton, Roger J. Pittock.
United States Patent |
5,157,525 |
Eaton , et al. |
October 20, 1992 |
Control of liquid crystal display visual properties to compensate
for variation in the characteristics of the liquid crystal
Abstract
The contrast or absolute brightness of a multiplexed LCD is
maintained at its preselected value using a feedback arrangement
which includes an LCD element functioning as a reference element.
The reference element is not used to display information but is
continually driven ON and OFF. The average transmissivity of the ON
and OFF states is determined and compared with a reference value,
the result of the comparison being used to control the voltage
levels of the drive waveforms applied to the LCD. By selecting
appropriate ratios between the ON and OFF times of the reference
element, the LCD can be operated to give optimum contrast, or may
have its absolute brightness varied. Control may be effected
remotely by reprogramming the microprocessor which determines the
timing of the drive waveforms.
Inventors: |
Eaton; Timothy J. (Chelmsford,
GB), Pittock; Roger J. (Salcutt, Near Maldon,
GB) |
Assignee: |
EEV Limited (Chelmsford,
GB)
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Family
ID: |
10665273 |
Appl.
No.: |
07/601,136 |
Filed: |
October 23, 1990 |
Foreign Application Priority Data
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Oct 27, 1989 [GB] |
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8924221 |
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Current U.S.
Class: |
345/87; 345/207;
345/690; 349/116; 349/33 |
Current CPC
Class: |
G09G
3/36 (20130101); G09G 2320/029 (20130101); G09G
2320/041 (20130101); G09G 2320/043 (20130101); G09G
2360/145 (20130101) |
Current International
Class: |
G09G
3/36 (20060101); G02F 001/133 () |
Field of
Search: |
;350/331T,332,333,345
;340/713,765,784,793,812 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0039523 |
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Apr 1981 |
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JP |
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0246014 |
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Oct 1987 |
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JP |
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0098636 |
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Apr 1988 |
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JP |
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0006927 |
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Jan 1989 |
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JP |
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2067812A |
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Jul 1981 |
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GB |
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2199439A |
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Jul 1988 |
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GB |
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2213303A |
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Aug 1989 |
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GB |
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Other References
Kahn et al.-"Temperature Dependence of Multiplexed TN-Liquid
Crystal Displays" Non-Emissive Electro-optic Display-Plenum
Press-New York-1976 pp. 289-297. .
Dickson et al., "Control Circuit For Liquid Crystal Cells" IBM
Technical Disclosure Bulletin-vol. 13, No. 11, Apr. 1971, p.
3517..
|
Primary Examiner: Miller; Stanley D.
Assistant Examiner: Duong; Tai V.
Attorney, Agent or Firm: Spencer, Frank & Schneider
Claims
We claim:
1. A liquid crystal display comprising;
a plurality of liquid crystal elements;
means for applying voltages across the elements;
photodetector means to detect the transmissivity of one of said
plurality of elements;
means to repetitively switch said one element at predetermined
times between a first state in which it exhibits a first level of
transmissivity and a second state in which it exhibits a second
level of transmissivity lower than the first level;
means coupled to the said photodetector means for obtaining a
signal corresponding to the mean transmissivity of the first and
second transitivities;
means for comparing the obtained signal with a reference signal
representative of a desired mean transmissivity;
means for producing the reference signal; and
means for adjusting the voltages applied across the plurality of
elements in dependence upon the difference between the obtained and
desired reference signals, whereby the mean transmissivity of the
liquid crystal elements is maintained at the desired mean
transmissivity.
2. A liquid crystal display as claimed in claim 1 in which the
elements are arranged such that respective first electrodes of a
number of elements are coupled to a single driver stage such that
all elements are individually addressable.
3. A liquid crystal display as claimed in claim 2 in which the
elements are disposed in a matrix of rows and columns.
4. A liquid crystal display as claimed claim 1 in which the means
for producing the reference value comprises first and second
reference element means, and further photodetector means, the first
reference element means being maintained in the first level of
transmissivity, the second reference element means being maintained
in the second level of transmissivity, the further photodetector
means being coupled to the first and second reference elements so
as to produce a signal representing the mean transmissivity of the
first and second elements; the signal so produced comprising the
said reference value.
5. A liquid crystal display as claimed in claim 1 in which the said
element is illuminated in reflective mode by a light source
disposed on the same side thereof as the said photodetector
means.
6. A liquid crystal display as claimed in claim 1 in which the said
element is illuminated in transmissive mode by a light source
disposed on the opposite side thereof from the said photodetector
means.
7. A liquid crystal display as claimed in claim 4 in which the
first and second reference elements are illuminated in reflective
mode by a light source disposed on the same side thereof as the
said photodetector means.
8. A liquid crystal display as claimed in claim 4 in which the said
element and the first and second reference elements are illuminated
in transmissive mode by a light source disposed on the opposite
side thereof from the said photodetector means.
9. A liquid crystal display as claimed in claim 1 in which the time
for which the said element is in the first state and the time for
which the said element is in the second state, are independently
adjustable.
10. A liquid crystal display as claimed in claim 1 in which the
time for which the said element is in the first state is
substantially the same as the time for which the said element is in
the second state.
11. A liquid crystal display as claimed in claim 9 in which the
times for which the said element is in the first and second states
respectively are adjustable so as to adjust either the absolute
level of brightness, or to provide maximum contrast of the
display.
12. A liquid crystal display as claimed in claim 9 comprising a
microcomputer to produce signals which determine the switching of
the elements of the liquid crystal display between first and second
states in which the time relationship between the first and second
states of the said elements is varied by reprogramming the
microcomputer.
Description
This invention relates to liquid crystal displays.
BACKGROUND OF THE INVENTION
Liquid crystal displays (LCDs) may be either directly driven or
multiplexed. In directly driven LCDs, each segment or element has
its own driver. In multiplexed LCDs, one driver drives a number of
elements. For multiplexed LCDs having large numbers of elements, a
matrix arrangement is commonly used, the matrix consisting of rows
and columns of conductors having elements disposed at the
intersection of each row and column conductor. The row and column
conductors are energised by multiple level driving waveforms. The
voltage levels of the waveforms are chosen according to the upper
and lower transmission voltage threshold values of the liquid
crystal and are conveniently generated by a resistive potential
divider. This allows the voltage levels to be adjusted in step with
each other by adjusting the voltage which is applied across the
potential divider, e.g. by hand tuning. Such adjustment is required
for initially setting up the display. Temperature-compensated LCDs
are known, in which a temperature - dependent voltage source is
included having a linear temperature voltage characteristic. Such
temperature compensation gives acceptable performance over a
limited temperature range, for example -5.degree. to 45.degree. C.
If it is desired to operate over a wider range, it would be
possible in principle to produce a voltage source having a
non-linear temperature characteristic matching that of the display,
but such a source would be considerably more complex and expensive
than one having a linear characteristic, and would require
calibration over the temperature range.
Another disadvantage of a temperature--controlled voltage source is
that the temperature responsive element is in general somewhat
remote from the display panel and has a different time--response
under rapid changes of temperature. This means that until the
temperature has stabilised, the contrast and legibility of the
display will be degraded.
SUMMARY OF THE INVENTION
This invention provides a liquid crystal display comprising a
plurality of liquid crystal elements; means for applying voltages
across the elements; photo-detector means to detect the
transmissivity of an element; and means for adjusting the voltages
applied across the cells in dependence on the transmissivity
detected.
The use of one of the elements of the display as a reference
element, and the measurement of its actual transmissivity allows
pre-selected optical properties e.g. contrast to be maintained even
if there are changes in the physical condition of the material of
the element e.g. due to temperature or ageing.
Voltages may be adjusted to obtain desired contrast or desired
brightness.
The LCD may include a light source to provide a reference
illumination of the element, and the photo-detector may be arranged
to detect the intensity of the light source retro-reflected through
the element.
BRIEF DESCRIPTION OF THE DRAWING
Preferred embodiments of the invention will now be described by way
of example with reference to the accompanying drawings in
which:
FIG. 1 shows a first embodiment of the invention in which a
multiplexed LCD display has a reference cell constructed as an
integral part of the display;
FIG. 2 shows a second embodiment utilising a discrete reference
cell;
FIG. 3 shows a third embodiment which is a modification of FIG. 1
in which a reference level is determined automatically;
FIG. 4 shows a fourth embodiment which is a modification of FIG. 2
in which the reference level is determined automatically.
FIG. 5 shows a fifth embodiment which is a modification of FIG.
3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a multiplexed Super Birefringent Effect (SBE) Liquid
Crystal Display (LCD) comprising a matrix LCD display panel 2
having a main array of pixels which operate in the standard
transflective or reflective mode in conjunction with either a rear
mounted transflector and backlight, or a rear mounted reflector 4.
A control pixel 1 is eclipsed from main view by the bezel and has a
front mounted reflector 9. The control pixel 1 is substantially
identical with the pixels of the main display. The control pixel is
illuminated from the rear by a light source 8 such as a LED. Light
from the light source passe through the control pixel 1, is
reflected by the reflector back through the control pixel and falls
on a photodetector 10. The control pixel 1 is driven alternately n
fields on and m fields off by a spare row/column combination of the
main horizontal 5 and vertical 6 LCD drive circuits. This is
achieved by suitably programming the system control micro-computer
7. It is necessary to drive the control pixel 1 such that no DC
bias be allowed to accrue across it.
The arithmetic mean of the output signal from the photodetector 10
is compared with a reference signal from a reference signal source
11 using a DC coupled Miller integrator-type comparator 12, the
time constant-determining components of which are selected such as
to effect satisfactory integration over the period of n+m fields.
The reference signal is chosen so as to correspond with the desired
transmission of the control pixel and hence of the main
display.
The output signal of the comparator is applied to the resistor
chain 13 which generates the reference voltages for the horizontal
and vertical driver circuits 5,6, thereby determining the V on and
V off voltages of both the control pixel and the main display.
To provide optimum contrast, the control pixel is driven such that
m=n=2, i.e. 2 fields on, and 2 fields off, and the reference signal
is chosen so as to correspond with 50% transmission of the control
pixel, and hence of the main display. This feedback ensures that V
on and V off are always maintained at values which produce optimum
contrast, even at extremes of temperature. This is because,
although the temperature/voltage characteristics are non-linear at
temperature extremes, the V on and V off voltages maintain their
relationship relative to the 50% transmission voltage.
Under certain circumstances it may be advantageous to increase or
decrease the absolute brightness of the display, even though this
means departure from the optimum contrast. This can be done by
changing the values of m and n such that the ratio between the ON
time and the OFF time of the control pixel is varied, the reference
value remaining constant. n and m are changed by reprogramming the
microcomputer, which is easily done and requires no additional
electrical connections. It also allows the brightness of the
display to be controlled remotely, using a databus to reprogram the
microcomputer.
Illumination of the reference pixel and monitoring of its
transmission may be effected by pulsed operation, for example 100
.s per field, in applications where power consumption is critical.
Sample and hold techniques are advantageously employed in such
arrangements.
In the embodiment of FIG. 1, as the reference pixel is an integral
part of the display, accurate stabilization of contrast or absolute
illumination will be maintained under forced heating or cooling of
the display, and for variations in the properties of the liquid
crystal itself.
A second embodiment of the invention is shown in FIG. 2. The main
difference between this and FIG. 1 is that the reference pixel is
not an integral part of the main display, but forms part of an
auxiliary LCD panel 14. The auxiliary LCD panel is made of the same
material and has identical electrical and optical properties as the
main display. Such an arrangement allows the photodetector 15 and
the light source 16 to be placed on opposite sides of the auxiliary
panel so as to operate the reference pixel in the transmission
mode. Otherwise, operation is identical to the FIG. 1
embodiment.
The embodiment of FIG. 3, shows a modification of the FIG. 1
embodiment in which the reference voltage is generated
automatically. The reference voltage source 11 of FIG. 1 is
replaced by second and third reference pixels 19, 20 and a second
photodetector 17. The second reference pixel 19 is driven so as to
be always hard ON, while the third reference pixel 20 is driven so
as to be always hard OFF e.g. by applying zero volts across it.
Light, which is conveniently obtained from the same light source 8
as that which illuminates the first reference pixel 1 of FIG. 1, is
passed through the second and third reference pixels and falls on
the second photodetector 17, which is preferably matched to the
first photodetector 10. The second detector thus produces an output
signal proportional to the sum of the best ON transmissivity and
the best OFF transmissivity. It can be adjusted to give the desired
reference value, namely half the sum of the ON transmissivity and
the OFF transmissivity, by any convenient means. For example, the
second and third reference pixels may each be constructed so as to
have half the area of the first reference pixel, the second
photosensor 17 may be half the area of the first photosensor 10, or
the Miller integrator comparator 12 may include scaling circuitry
e.g. a potential divider to reduce the value of the signal applied
to it from the second photo sensor.
This arrangement is particularly advantageous as it requires no
setting up or adjustment, even when different types of liquid
crystal are used, the reference value always being set to give the
optimum value for the particular liquid crystal being used.
The embodiment of FIG. 4 is likewise a modification of FIG. 2, and
like FIG. 3, has second and third reference pixels 19,20, the
second 19 being always hard ON, the third 20 being always hard
OFF.
These additional reference pixels are preferably, but not
necessarily, constructed in the same auxiliary LCD panel as the
first reference pixel. Operation is otherwise the same as the
embodiment of FIG. 3.
The embodiment of FIG. 5 is a modification of FIG. 3. In this
embodiment, the light source 8 of FIG. 3 is not used; instead the
ambient light incident on the front of the display is allowed to
pass through the first, second and third reference elements.
Otherwise operation is identical with the FIG. 3 embodiment.
While the description refers to light, this invention is not
restricted to visible light, but also encompasses non-visible light
e.g. ultra-violet and infra-red.
Further, while the invention has been described with particular
reference to a matrix array, the invention is not restricted to the
particular embodiments described. It is equally applicable to
multiplexed LCDs in the form of alphanumeric displays, and
indicators, or to non-multiplexed LCDs.
* * * * *